This invention relates to a method for concentrating and removing hydrogen sulfide from a contaminated gas mixture containing hydrogen sulfide and carbon dioxide, such as may be found in contaminated natural gas or the contaminated product gas from coal gasification or from the partial oxidation of petroleum.
Numerous processes for removing hydrogen sulfide and carbon dioxide from gaseous mixtures involving physical absorption are well known. Kutsher et al, U.S. Pat. No. 3,362,133, discloses a process for removing hydrogen sulfide and carbon dioxide by physical absorption in the dialkylether of a polyalkylene glycol and then flashing the solvent at a pressure substantially lower than the absorber to release any co-absorbed CO.sub.2, which is recycled to the absorber. The flashing step separates CO.sub.2 from the solvent, but any recycle can be effected only by compressing the CO.sub.2 before it can be recycled into the absorber which is at a substantially higher pressure than the flashed CO.sub.2. Tjoa et al., U.S. Pat. No. 3,767,766, uses a process similar to Kutsher and shows passing the solvent through an expansion valve to flash and vaporize dissolved CO.sub.2 and/or hydrocarbons which may be recycled through a compressor to the absorber.
Hill, U.S. Pat. No. 3,989,811, shows a sulfur recovery process in which an absorbent containing H.sub.2 S and CO.sub.2 is heated and then stripped of H.sub.2 S and CO.sub.2 in a low pressure stripping column to regenerate the solvent. The stripped vapors are passed to a second H.sub.2 S absorber to separate CO.sub.2 from the H.sub.2 S, and the CO.sub.2 is vented.
Child et al, U.S. Pat. No. 4,520,176, shows a process for removing acid gases including CO.sub.2, H.sub.2 S, and COS, from a raw synthesis gas feed stream by physical absorption. The hydrogen sulfide rich absorbent is stripped in low pressure columns using a portion of the H.sub.2 -rich product gas stream as an optional stripping gas to regenerate purified solvent. The gas stream leaving the stripper is introduced into underground reservoirs to facilitate petroleum recovery.
The conventional processes for separating hydrogen sulfide and carbon dioxide from gaseous mixtures, as represented by the foregoing, are limited by the low pressure techniques of separating any co-absorbed carbon dioxide from the solvent rich in absorbed hydrogen sulfide, such as by expansion and flashing or by low pressure stripping columns. Using the conventional methods, large and costly compressors are required to effect a recycle back to an H.sub.2 S absorber which necessarily can be operated only at a pressure substantially higher than the pressure in the flashing separation vessel.
An object of this invention is to overcome the limitations of the conventional processes and provide for an efficient separation of any co-absorbed carbon dioxide in the hydrogen sulfide rich solvent leaving the H.sub.2 S absorber. It is a further object of this invention to provide for an efficient separation and removal of said co-absorbed CO.sub.2 without the need for large and expensive compressors to recycle any stripped CO.sub.2 into the high pressure H.sub.2 S absorber. It is a further object of this invention to provide for the use of the portion of the product gas from the contemplated gas cleanup as the stripping gas in the CO.sub.2 stripper for removing any co-absorbed CO.sub.2 from the hydrogen sulfide rich solvent. It is yet a further object of this invention to adjust the portion of product gas used in the CO.sub.2 stripper so as to provide for a H.sub.2 S concentration higher than conventional methods in the product Claus gas for use in a sulfur recovery unit.